Fuel assembly with outer channel including reinforced sidewall

ABSTRACT

The fuel assembly includes at least one fuel rod and an outer channel with four sidewalls surrounding the fuel rod, the outer channel having a configuration based on a position of the fuel assembly within a core of the nuclear reactor, wherein at least a first select sidewall, of the four sidewalls of the outer channel, is a reinforced sidewall, the remaining sidewalls of the outer channel, other than the at least a first select sidewall, are non-reinforced sidewalls, the at least a first select sidewall being in a controlled location that faces and is directly adjacent to a control blade that is to be utilized in the nuclear reactor, wherein an entirety of the reinforced sidewall as a whole is at least one of thicker and made from a material that is more resistant to radiation-induced deformation as compared to an entirety of the non-reinforced sidewalls.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.16/850,128, filed Apr. 16, 2020, which is a divisional of U.S. patentapplication Ser. No. 15/069,302, filed Mar. 14, 2016, which is adivisional of U.S. patent application Ser. No. 12/843,037, filed Jul.25, 2010, the contents of each of which are incorporated by reference inits entirety.

BACKGROUND

As shown in FIG. 1, a conventional fuel assembly 10 of a nuclearreactor, such as a Boiling Water Reactor (BWR), may include an outerchannel 12 surrounding an upper tie plate 14 and a lower tie plate 16. Aplurality of full length fuel rods 18 and/or part length fuel rods 19may be arranged in a matrix within the fuel assembly 10 and pass througha plurality of spacers (also known as spacer grids) 15 axially spacedone from the other and maintaining the rods 18, 19 in the given matrixthereof. The fuel rods 18 and 19 are generally continuous from theirbase to terminal, which, in the case of the full length fuel rod 18, isfrom the lower tie plate 16 to the upper tie plate 14. Outer channel 12encloses the fuel rods 18/19 within the assembly 10 and maintains wateror other coolant flow within assembly 10 about fuel rods 18/19 and incontact with the fuel rods 18/19 to facilitate heat transfer from thefuel to the coolant. Outer channel 12 is traditionally uniform inmechanical design and material for each other assembly 10 provided to aparticular core, to aid in assembly design standardization andmanufacturing simplicity. Outer channel 12 may be fabricatedconventionally of a material compatible with the operating nuclearreactor environment, such as a Zircaloy-2.

As shown in FIG. 2, a conventional reactor core, such as a BWR core, mayinclude a plurality of cells 40 in the reactor core. Each cell mayinclude four fuel assemblies 10 having adjacent fuel channels 12. Otherfuel assemblies 10 may be placed in the reactor core outside of cells 40and not adjacent to control blades. The fuel assemblies 10 in FIG. 2 areshown in section to illustrate control blades 45, which areconventionally cruciform-shaped and movably-positioned between theadjacent surfaces of the fuel channels 12 in a cell 40 for purposes ofcontrolling the reaction rate of the reactor core. Conventionally, thereis one control blade 45 per cell 40. As a result, each fuel channel 12has two sides adjacent to the control blade 45 and two sides with noadjacent control blade.

The control blade 45 is formed of materials that are capable ofabsorbing neutrons without undergoing fission itself, for example,boron, hafnium, silver, indium, cadmium, or other elements having asufficiently high capture cross section for neutrons. Thus, when thecontrol blade 45 is moved between the adjacent surfaces of the fuelchannels 12, the control blade 45 absorbs neutrons which would otherwisecontribute to the fission reaction in the core. On the other hand, whenthe control blade 45 is moved out of the way, more neutrons will beallowed to contribute to the fission reaction in the core.Conventionally, only a fraction of all control blades 45 within a corewill be exercised to control the fission reaction within the core duringan operating cycle. As such, only a corresponding fraction of fuelassemblies will be directly adjacent to an extended control blade, or“subject to control,” during an operating cycle.

After a period of time, a fuel channel 12 may become distorted as aresult of differential irradiation growth, differential hydrogenabsorption, and/or irradiation creep. Differential irradiation growth iscaused by fluence gradients and results in fluence-gradient bow.Differential hydrogen absorption is a function of differential corrosionresulting from shadow corrosion on the channel sides adjacent to thecontrol blades 45 and the percent of hydrogen liberated from thecorrosion process that is absorbed into the fuel channel 12; thisresults in shadow corrosion-induced bow. Irradiation creep is caused bya pressure drop across the channel faces, which results in permanentdistortion called creep bulge. As a result, the distortion (bow andbulge) of the fuel channel 12 may interfere with the movement of thecontrol blade 45. Channel/control blade interference may causeuncertainty in control blade location, increased loads on reactorstructural components, and decreased scram velocities. Conventionally,if channel/control blade interference has become severe, the controlblade is declared inoperable and remains fully inserted.

SUMMARY

Example embodiments are directed to fuel assemblies useable in nuclearreactors and methods of optimizing and fabricating the same. Exampleembodiment fuel assemblies include an outer channel having a physicalconfiguration determined based on a position of the fuel assembly withina core of the nuclear reactor, such as the position of the fuel assemblywith respect to a control blade in the nuclear reactor that will be usedto control core reactivity. When example embodiment fuel assemblies areto be directly adjacent to an inserted control blade, the outer channelmay be thickened, reinforced, and/or fabricated of a material moreresistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, andVB, so as to reduce or prevent distortion of the channel against thecontrol blade and interfering with operation of the same. When exampleembodiment fuel assemblies are not in a controlled location, the outerchannel may be thinned so as to increase water volume and reactivity inthe assembly. As such, a reactor core including example embodiment fuelassemblies will include fuel assemblies having unique outer channels, inthickness, material, etc., unlike conventional power reactor cores.

Example methods of configuring fuel assemblies include determiningoperational characteristics of the fuel assembly, such as the likelihoodthat the fuel assembly is controlled via control blade insertion in thenuclear reactor in a current or future fuel cycle, and physicallyselecting or modifying the outer channel of the fuel assembly basedthereon. For example, if the fuel assembly is in a controlled locationduring the fuel cycle, the outer channel may be fabricated of a materialmore resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF,or VB, and/or thickened. Or, for example, if the fuel assembly is not ina controlled location, the outer channel may be approximately 20 mils(thousandths of an inch) or more thinner than outer channels ofconventional fuel assemblies. Example methods are useable with or mayfurther include configuring outer channel characteristics in order tomeet desired neutronic properties of the fuel assembly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a conventional fuel assembly having anouter channel.

FIG. 2 is an illustration of a conventional cell grouping in a reactorcore with a cruciform control blade.

FIG. 3 is a flow chart of an example method of optimizing fuel channelsin fuel assemblies.

FIG. 4 is a schematic illustration of a core section containing exampleembodiment fuel bundles optimized in accordance with example methods.

DETAILED DESCRIPTION

Hereinafter, example embodiments will be described in detail withreference to the attached drawings. However, specific structural andfunctional details disclosed herein are merely representative forpurposes of describing example embodiments. The example embodiments maybe embodied in many alternate forms and should not be construed aslimited to only example embodiments set forth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected,” “coupled,” “mated,” “attached,” or “fixed” to anotherelement, it can be directly connected or coupled to the other element orintervening elements may be present. In contrast, when an element isreferred to as being “directly connected” or “directly coupled” toanother element, there are no intervening elements present. Other wordsused to describe the relationship between elements should be interpretedin a like fashion (e.g., “between” versus “directly between”, “adjacent”versus “directly adjacent”, etc.).

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the language explicitlyindicates otherwise. It will be further understood that the terms“comprises”, “comprising,” “includes,” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, “channel,” “outer channel,” and the like are defined inaccordance with the conventional fuel assembly structures shown anddescribed in FIG. 1 as element 12, subject to the modificationsdiscussed hereafter. As used herein, “distortion” or “channeldistortion” includes both channel bow and channel bulge in nuclear fuelassemblies that may cause interference with control blade operation.

It should also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures ordescribed in the specification. For example, two figures or steps shownin succession may in fact be executed in parallel and concurrently ormay sometimes be executed in the reverse order or repetitively,depending upon the functionality/acts involved.

The inventors of the present application have identified severalpotential fuel channel characteristics and/or modifications to reduce orprevent fuel channel distortion. The inventors of the presentapplication have further identified the effect these characteristics, incombination with other fuel assembly parameters, have on whole coreperformance. Example embodiments and methods discussed below uniquelyaddress these previously-unidentified effects to achieve severaladvantages, including improved core performance, increased energygeneration, reduced control blade error, materials conservation, and/orother advantages discussed below or not, in commercial nuclear powerplants, while departing from total fuel channel uniformity purposefullyused in conventional commercial nuclear power plants.

Example Embodiments

Example embodiment fuel assemblies include fuel channels with optimizedphysical properties. Example embodiment fuel assemblies may include oneor more channel characteristics to decrease fuel channel distortion. Forexample, the fuel channel may be thickened in its shortest dimension orreinforced with additional material. The thicker or reinforced fuelchannel has greater resistance to distortion from differentialirradiation growth, differential hydrogen absorption, and/or irradiationcreep experienced in operating nuclear reactor environments. The percentreduction in deformation is approximately proportional the percentageincrease in channel thickness.

Or, for example, materials may be used in the channel that are resistantto distortion. For example, Zircaloy-4, a known zirconium alloyexcluding nickel, may replace Zircaloy-2, which contains nickel. Thereduced nickel content in Zircaloy-4 reduces differential hydrogenabsorption and resultant channel bow. Other materials more resistant todeformation than Zircaloy-2 may additionally be used in whole or in partin addition to Zircaloy-4. For example, additional materials moreresistant to deformation than Zircaloy-2 are described in co-pendingapplication Ser. No. 12/153,415 “Multi-layer Fuel Channel and Method ofFabricating the Same,” incorporated herein by reference in its entirety.That document discloses alloys hereinafter called “NSF” having about0.6-1.4% niobium (Nb), about 0.2-0.5% iron (Fe), and about 0.5-1.0% tin(Sn), with the balance being essentially zirconium (Zr) and alloyshereinafter called “VB” having about 0.4-0.6% tin (Sn), about 0.4-0.6%Fe, and about 0.8-1.2% chromium (Cr), with the balance being essentiallyzirconium (Zr).

Other configurations for decreasing fuel channel distortion are useablewith example embodiment fuel assemblies. Example embodiment fuelassemblies may use multiple mechanisms in combination to further reducefuel channel distortion. Configurations and fuel channel characteristicsin example embodiment fuel channels may be selected in accordance withexample methods, discussed in the following section.

Example embodiment fuel assemblies may further include channelcharacteristics that improve fuel neutronic characteristics, decreasematerial usage and costs, and/or improve other fuel assembly parameters.Such characteristics may include, for example, a thinner channel thatpermits greater water volume and neutron moderation within exampleembodiment fuel assemblies. The thinner channel may consume lessmaterial in fabrication and improve fuel assembly reactivity, heattransfer characteristics, etc.

Example embodiment fuel assemblies having thicker, reinforced, and/orthinner channels, different alloys, or other channel modification may beused instead of conventional fuel assemblies having standardizedchannels throughout an entire core. Example embodiment fuel assembliesmay thus significantly improve performance of a core including exampleembodiment fuel assemblies and/or reduce fuel resource consumption. Forexample, thinning the channels of 75% of the fresh conventional fuelassemblies for a particular fuel cycle by approximately 20 mils (20thousandths of an inch) in the thinnest dimension may result in areduction in volume of approximately 16,500 in³ zirconium alloy used. Inthe same example, assuming 8 channels would not need to be replacedbecause they include channel mechanisms to decrease fuel channeldeformation, an additional ˜2,000 in³ zirconium alloy volume may besaved. In the same example, assuming 8 channels are not needed to befabricated because 8 fewer fuel assemblies are required in a fuel cyclewith fuel savings from channel characteristics that improve fuelneutronic characteristics of example embodiment fuel assemblies, anadditional ˜2,000 in³ zirconium alloy volume may be conserved. Thus,example embodiment fuel assemblies, having different channelcharacteristics selected and implemented in accordance with examplemethods discussed below, may result in significant materials savings andimproved core performance.

Example Methods

As discussed above, increasing channel thickness decreases water volumeand overall reactivity of an assembly having a thicker channel. Lowerreactivity results in less optimal fuel usage and less power productionin a nuclear core of a nuclear power reactor. Increasing channelthickness further increases costs of fuel assemblies having thickerchannels. Increasing channel thickness also reduces the risk and/ormagnitude of channel distortion and interference with control bladefunction. Decreasing channel thickness has a generally opposite effectof increasing water volume and overall reactivity of an assembly havinga thinner channel, while also increasing distortion likelihood.Zircaloy-4 has similar fluence bow and creep bulge characteristicscompared to Zircaloy-2. Zircaloy-4, however, resists channel bow causedby differential hydrogen absorption. NSF and VB are additionallyresistant to other forms of bow and bulge causing channel deformation.

Example methods uniquely leverage the above advantages and disadvantagesof fuel channel modification to reduce or prevent channel distortionwhile minimizing negative effects on fuel economy, control bladefunction, and other core performance metrics. As shown in FIG. 3,example methods include an operation S100 of determining fuel assemblycharacteristics, including whether a fuel assembly is placed or will belocated in a cell such that the fuel assembly will be directly adjacentto a control blade that will be operated in a current and/or future fuelcycle to control the fission reaction in the core. A fuel assemblypositioned directly adjacent to a control blade that is likely to beexercised to control the fission reaction is herein defined as a“controlled fuel assembly” or in a “controlled location,” because it ismost subject to control blade negative reactivity and most likely toaffect control blade performance. The determination of whether a fuelassembly is subject to control may be based on one or more fuel assemblyoperational characteristics that determines placement/position of thefuel assembly within the reactor core over one or more fuel cycles, inaddition to overall plant characteristics such as core size, thermalpower rating, etc.

For example, an operational characteristic may be reactivity of the fuelassembly. Reactivity determines the degree to which the fuel cancontribute to the fission chain reaction during power operations.Reactivity is directly controllable with control blade insertion, due tothe blades' neutron-absorbing properties. As such, fuel with higherreactivity may be placed in controlled locations to enhance core-wideoverall control of the neutron chain reaction. Similarly, fuel withlower reactivity may be less likely to be subject to control.

Although location with regard to utilized cruciform control blades isdescribed in connection with example embodiments and methods, it isunderstood that other sources of negative reactivity may additionally beaccounted for in example methods and embodiments. For example, proximityto burnable poisons or proximity to a control rod present in some plantdesigns may be accounted for by determining operational characteristicsof the fuel assembly that determine the likelihood that the fuelassembly will be placed in that proximity.

Controlled locations may also be determined in S100 by known coremodeling and mapping methods and software. For example, a program mayreceive input of several fuel assembly operational characteristics forseveral fuel assemblies and determine an optimum core configuration withcorresponding fuel assembly positions. Because example methods andembodiments may themselves affect fuel assembly operationalcharacteristics as discussed below, such known core modeling and mappingmethods may be alternatively and repetitively executed before andfollowing fuel assembly modification in example methods to ensureoptimized core performance.

Following the determination in S100, one or more fuel assembly channelsare configured based on the position determination. The configuringgenerally increases assembly reactivity, decreases distortion potential,and/or reduces material consumption in the configuredassembly/assemblies.

If it is determined from S100 that the assembly will be placed in a celladjacent to an employed control blade, i.e., subject to control, then afirst configuration S210 is pursued. S210 configures the assemblychannel to reduce or eliminate channel distortion during poweroperations. For example, in S210, channel thickness may be increased byseveral hundredths of an inch or more to ensure decreased channeldistortion. The degree of thickening may further be based on decreasedreactivity or other operational characteristics desired of the assemblyduring operation in the nuclear reactor core. Or, channel thickness maybe increased or the channel may be reinforced on only a side or walldirectly adjacent to the control blade that will be operated, whileremaining fuel channel sides may be unmodified or modified in accordancewith S220.

Additionally, or in the alternative, in S210, the channel may befabricated out of a material more resistant to distortion thanZircaloy-2, including shadow-corrosion-bow-resistant Zircaloy-4, orfluence-gradient-bow and/or creep-bulge-resistant NSF or VB. In thisway, only assemblies determined to be at a position benefitting from athicker or reinforced channel or a channel including Zircaloy-4, NSF,and/or VB, such as a controlled assembly likely to be placed in a celladjacent to an employed control blade, are configured with channelfeatures that decrease or eliminate distortion while leveraging othercharacteristics such as reactivity or fabrication expense. Further,because assemblies in a controlled core position typically possesshigher excess reactivity, a thicker or reinforced channel that maydecrease reactivity is not a significant disadvantage for the overallcore reactivity; indeed, such reactivity-decreasing configuration mayaid in balancing core power production and/or simplifying control bladeoperations.

If it is determined from S100 that the assembly will be placed in anuncontrolled core position, such as an edge position in the core oradjacent to a control blade that will not be utilized, then a secondconfiguration S220 is pursued. S220 configures the assembly channel toincrease fuel assembly neutronic characteristics for the assembly in theoperating core and decrease manufacturing burden in fabricating theassembly, without regard to distortion risk. For example, in S220,channel thickness may be decreased by several hundredths of an inch ormore to increase water or moderator volume in the assembly, therebyincreasing reactivity and fuel usage in the assembly. Reducing channelthickness in S220 further decreases an amount of expensive zirconiumalloy or other channel material required to fabricate the assembly. InS220, assembly channel thickness may be reduced by a margin that takesinto account the increased reactivity; the channel may be thinned suchthat the assembly has a determined or desired reactivity or otheroperational property when in use in the nuclear reactor core. In thisway, a core may contain fuel assemblies with several different, uniquechannel thicknesses and other characteristics as determined in S210 andS220.

Assemblies may be configured in S210 and S220 in several differentmanners and timeframes. For example, the configuring in S210 and S220may be selecting a pre-existing assembly or ordering an assembly havingthe configuration determined in S210 and S220, by a power plantoperator, for insertion or re-insertion during an upcoming fuel cycle inthe nuclear reactor core. Alternatively, the configuring in S210 andS220 may be a physical fabricating or modifying of the fuel assembly tomatch the configuration determined in S210 and S220 by a fuel assemblymanufacturer or refitter, for example.

Example methods including S100 and S210/S220 may address fuel assemblylocation and configuration for use in an immediately approaching fuelcycle, a future fuel cycle, and/or multiple fuel cycles. For example,S100 may determine that a fuel assembly will be in a controlled positionadjacent to an employed control blade in a first fuel cycle, and thesame or later analysis may determine that the fuel assembly will berelocated to a position away from a control blade in a second layer fuelcycle. The assembly may be configured under S210 for the first cycle,and then reconfigured under S220 for the second cycle. Suchreconfiguring may include re-channeling the fuel assembly by removingand replacing the channel used in the first fuel cycle with a channelhaving the configuration determined in S220 for use in the second fuelcycle. Similarly, a reverse determination may result in the reverseconfiguration. Or, for example, S100 may determine, based on multi-cycleoperating parameters, that a particular fuel assembly will not be placedin a controlled location in its lifetime. Configuration of the assemblymay then proceed under S220, without further modification of theassembly during its lifetime in the reactor.

FIG. 4 is an illustration of an example reactor core 400 containingexample embodiment fuel assemblies 100 and 200 modified in accordancewith example methods. As shown in FIG. 4, four example embodimentassemblies 100 are in controlled locations about a control blade 45 athat is anticipated to be used to control the fission chain reaction inthe core. According to example methods, assemblies 100 about blade 45 ahave channels 120 configured in accordance with S210. For example,channels 120 may be thickened, reinforced, and/or fabricated of amaterial more resistant to deformation than Zircaloy-2. Or, for example,only select sides or walls 120 b directly adjacent to control blade 45 amay be configured in accordance with S210, including being thickened,reinforced, and/or fabricated of a material more resistant todeformation than Zircaloy-2. Other walls 120 a may be unmodified orthinned and/or fabricated of a material equally or less resistant todeformation than Zircaloy-2, in accordance with S220.

Example assemblies 200, adjacent to blade 45 b that is not to beoperated during the fuel cycle or adjacent to no control blade, may beconfigured in accordance with S220. For example, channels 121 inassemblies 200 may be thinned and/or fabricated of a material equally orless resistant to deformation than Zircaloy-2.

Example methods including S100 and S210/S220 may be executed for eachassembly to be placed within a core. Alternatively, example methods maybe executed only with respect to particular assemblies in order tooptimize core operating characteristics. For example, if example fuelassembly channel configuring methods are used in conjunction with otherknown core configuration methods, the calculated or desired fuelassembly locations and characteristics may require no fuel assemblychannel configuring or reconfiguring as in S210 or S220.

Example methods may be used as an integral part of core design or as aseparate step performed alternatively and/or iteratively with otherknown methods of core design. For example, a known core design programmay output a core map using fuel assembly characteristics with fuelhaving uniform channel properties. Example methods including S100 andS210/S220 may then be performed on some or all fuel assemblies involvedin the map, changing their operational characteristics. The core designprogram may then be re-executed with the modified fuel assemblycharacteristics, and this alternating core configuring between exampleand known methods may continue until no further optimization is possibleor desired. Or, example methods may be used as an integral part ofotherwise known core design methods, treating reactivity, bowlikelihood, and other fuel assembly parameters affected by channelconfiguring in S210 and S220 as additional variables in the core designprocess.

Example embodiments and methods thus being described, it will beappreciated by one skilled in the art that example embodiments may bevaried through routine experimentation and without further inventiveactivity. Variations are not to be regarded as departure from the spiritand scope of the example embodiments, and all such modifications aswould be obvious to one skilled in the art are intended to be includedwithin the scope of the following claims.

1. (canceled)
 2. A fuel assembly for use in a nuclear reactor, the fuelassembly comprising: at least one fuel rod; and an outer channel withfour sidewalls surrounding the fuel rod, the outer channel having aconfiguration based on a position of the fuel assembly within a core ofthe nuclear reactor, wherein at least a first select sidewall, of thefour sidewalls of the outer channel, is a reinforced sidewall, theremaining sidewalls of the outer channel, other than the at least afirst select sidewall, are non-reinforced sidewalls, the at least afirst select sidewall being in a controlled location that faces and isdirectly adjacent to a control blade that is to be utilized in thenuclear reactor, wherein an entirety of the reinforced sidewall as awhole is at least one of thicker and made from a material that is moreresistant to radiation-induced deformation as compared to an entirety ofthe non-reinforced sidewalls.
 3. The fuel assembly of claim 2, whereinthe at least a first select sidewall includes two select sidewalls ofthe fuel assembly that are in a controlled location such that the twoselect sidewalls face and are directly adjacent to a control blade thatis to be utilized in the reactor core, the two select sidewalls beingreinforced sidewalls that are adjacent to each other on the fuelassembly.
 4. The fuel assembly of claim 3, wherein the non-reinforcedsidewalls do not face a control blade that is to be utilized in thenuclear reactor.
 5. The fuel assembly of claim 2, wherein, thenon-reinforced sidewalls are made from a zirconium alloy that is equalto or less resistant to radiation-induced deformation than Zircaloy-2,the reinforced sidewalls are made from a zirconium alloy that is moreresistant to radiation-induced deformation than Zircaloy-2, thereinforced sidewalls being made from at least one of Zircaloy-4, NSF andVB.
 6. The fuel assembly of claim 2, wherein the reinforced sidewall is20 mils or more thicker on average than the non-reinforced sidewalls. 7.A reactor core in a commercial nuclear power plant, the reactor corecomprising: the fuel assembly of claim 2, which is a first fuel assemblywhere the at least a first select sidewall is all four sidewalls of thefirst fuel assembly; and at least a second fuel assembly in anon-controlled location of the reactor core, the at least a second fuelassembly being at least one of positioned adjacent to control bladesthat are not expected to be utilized, positioned in an area of reactorcore that has a lower relative reactivity than the rest of the reactorcore, and positioned at an edge location within the reactor core, allfour sidewalls of the at least a second fuel assembly havingnon-reinforced sidewalls.